1. Field of the Invention
The present invention relates to a light amount adjusting apparatus provided in a digital camera or the like and to an image pickup apparatus equipped with the same.
2. Related Background Art
Recently, digital cameras capable of taking images in memory without using films have sprung into wide use. Among such digital cameras is one endowed with a function by which it puts the diaphragm blades in and out of a light beam according to the object illuminance, thereby varying the amount of light reaching the image pickup device. In this process, it is general practice to drive the diaphragm blades by a dedicated electromagnetic actuator to perform light amount adjustment. Further, there exists a device often used in a digital camera, a video camera or the like with a built-in CCD. This device adopts an exposure structure in which an ND filter adapted to reduce the amount of light passing therethrough is bonded to a part of the diaphragm blades and in which, when the size of the diaphragm aperture formed by the diaphragm blades becomes smaller than a certain size, the ND filter is inserted into the diaphragm aperture.
The diaphragm blades each are generally formed as a shutter unit; in a typical example of the construction of such a shutter unit, a shutter blade is incorporated into a base plate, and a partition plate is placed thereon; further, a diaphragm blade is placed thereon before covering the whole with a shutter cover. An actuator for driving the shutter blade and the diaphragm blade is arranged on the opposite side in the light axis direction of the shutter blade and the diaphragm blades with respect to the base plate.
When such a photographing apparatus, e.g., a digital camera, is used in a hostile environment, such as one with high humidity, dew condensation may occur within the camera, which leads to a problem in terms of internal function. In view of this, various countermeasures have been taken; for example, a ventilation hole for letting air into and out of the camera body is provided to thereby restrain an increase in internal humidity, or, as disclosed in Japanese Patent Application Laid-Open No. 2000-171874, there is provided dew condensation detection means for detecting dew condensation within the camera body.
However, the method in which a ventilation hole for preventing dew condensation is provided has a problem in that, as a result of the general reduction in camera size, there are limitations regarding the space for providing the ventilation hole, and there is little possibility of effectively arranging the ventilation hole. Further, with the tough competition in cost, there is a demand for a reduction in cost; under the circumstances, it is rather difficult to arrange such a dew condensation detection sensor, and it is also becoming increasingly difficult to secure the space for the sensor.
Further, in the case in which the shutter unit as mentioned above is formed, when dew condensation occurs and the diaphragm blades and the partition plate are brought into close contact with each other, the drive load of the diaphragm blades increases, so that it is necessary to increase the drive force by, for example, enlarging the actuator. However, due to the limitation in terms of space as a result of the general reduction in camera size, it would be impossible to realize a construction in which the actuator for the diaphragm blades is enlarged in order to operate them with a drive force larger than the force with which the diaphragm blades and the partition plate are held in close contact with each other.
Apart from this, there is a method according to which hemispherical projections are provided on the diaphragm blades at positions where the diaphragm blades and the partition plate come into contact with each other with a view to preventing the diaphragm blades and the partition plate as much as possible from close contact with each other. FIGS. 15A and 15B show a diaphragm blade 1′ provided with hemispherical projections. FIG. 15A is a front view of the diaphragm blade 1′, and FIG. 15B is a rear view of the diaphragm blade 1′. The diaphragm blade 1′ is equipped with an opening 1a′, an ND filter 9 covering the same, and hemispherical projections 1b′ provided on one side of the diaphragm blade 1. Due to the hemispherical projections 1b′, the contact area between the diaphragm blade 1′ and the partition plate 2 is reduced, so that even in a high humidity condition, the force with which the partition plate 2 and the diaphragm blade 1′ are held in close contact with each other can be reduced, and the diaphragm blade 1′ can be operated smoothly. However, in this case also, the diaphragm blade may bend or fall, making it difficult to completely eliminate close contact between the diaphragm blade and the partition plate. Further, as the reduction in size advances, there is a demand for elimination of even the space corresponding to these projections.